1. Field of the Invention
This invention relates to a method and apparatus for wireless communications and, more particularly, to adaptive power control for CDMA communication systems.
2. Description of the Related Art
Code division multiple access (CDMA) is one of several techniques for multiplexing wireless users. In CDMA systems, users are multiplexed over the same wireless channel by using distinct spreading codes rather than by using orthogonal frequency bands as in frequency division multiple access (FDMA) systems or by using orthogonal time bands as in time division multiple access (TDMA) systems. In CDMA systems, all the users can transmit simultaneously using the entire available transmission spectrum.
Power control is an important part of these systems. Such power control can have a substantial impact on the capacity and apparent quality of service of the CDMA system. The need for power control in these systems arises at least from the need to mitigate intercell interference that arises from frequency reuse. In CDMA systems, a further need for power control arises for minimizing intracell interference. For current code-division multiple-access (CDMA) cellular systems (IS-95), transmission power is adapted so as to maintain the same received desired power level from all mobiles. When a wireless transmitter is provided with suitable channel state information (CSI), adaptation of the transmission power in response to channel variations can be a powerful and efficient fading mitigation technique in wireless communication systems.
In recent years, there has been considerable interest in multicarrier (MC) modulation techniques for high bit rate applications in fading channels. MC-CDMA modulation, a combination of frequency domain spreading and multicarrier modulation, is employed to achieve frequency diversity and multiple access operation. MC-CDMA systems generally are divided into two types: the first type encodes the original data sequence for a user via a spreading sequence and then a different carrier with each chip, and the second type spreads serial-to-parallel converted data sequences using a given spreading code and then modulates a different carrier with each of the data sequences. Se, for example, S. Hara et al. “Overview of multicarrier CDMA,” IEEE Communications Magazine, pp. 126-133 (December 1997). A conventional MC-CDMA transmitter allocates the available transmission power uniformly over all subcarriers.
In MC-CDMA systems, a form of power control or adaptation proposed wherein the transmitter uses only those subcarriers for which channel gains are higher than the given threshold level and truncates or turns off the other subcarriers while applying maximal ratio combining (MRC) at the receiver. See Zhu et al, “Performance of MC-CDMA systems using controlled MRC with power control in Raleigh fading channel,” Elec. Lett., Vol. 36, pp. 752-53 (April 2000). This approach, however, results in transmission outages when the channel gains of all subcarriers are below the threshold level, a condition that is unacceptable for most communication traffic that is intolerant for any number of reasons to delay. A power allocation algorithm for MC-CDMA with a projection matrix based receiver was proposed where it was described that the optimal power allocation coefficients are the components of the eigenvector which corresponds to maximum eigenvalue of modified projection matrix orthogonal to the interference signal space. See Zhu et al., “Power allocation algorithm in MC-CDMA,” Proc. IEEE ICC, pp. 931-35 (May 2002). This adaptation method requires the knowledge of all users' spreading codes and channel responses, which increases the system complexity especially for larger number of users, making the implementation infeasible for realistically sized systems expected in practice.
Although various adaptive power control techniques have been proposed for MC-CDMA systems, none have presented a practical solution that can be employed in currently expected communication systems based user quality of service demands and the size of the user community.